VAST, damp, and noisily abuzz, the ecosystem of the rain forest in its delicate yet powerful rhythms is understood as dangerous. But at night, few scientists or natives ever defy the darkness to venture into the jungle tangle.

Yet in the race to understand the world's rapidly disappearing rain forests, biologist Louise Emmons has chosen not to ignore that half of the life of the forest that begins after dark.

She is legendary among rain forest experts for her meticulous observations. They marvel at how her scientific obsession seems like an invisible armor against the hazards of predators, disease, and falling branches.

Her radical style of "bottom-up" research, tracking a single animal during its whole waking day or night, has filled vacuums of knowledge about all kinds of species - "[with] the kind of thing you put in textbooks," says Patricia Wright, an associate professor of biological anthropology at Duke University who worked with Emmons in Peru's Manu National Park. It is a style that many scientists never even consider because it is so physically demanding, she says.

Dr. Emmons herself explains that a typical workday means locating radio signals from animals she has previously collared. Carrying a camera, notepad, radio receiver, binoculars, gun, and rain gear, she arrives at the den before the animal awakes, following it until it returns.

"I think you can learn a lot more by essentially seeing everywhere an animal goes," she says.

"I'm just walking up and down while it's looking for food. If it's hungry and there isn't much food available, it can run all day long. It's actually extremely interesting because you really get to feel firsthand what it's like to be a little animal out there without a fruit tree ... maybe a tree shrew that only weighs 50 grams [1.75 ounces] will be running several kilometers a day just looking for something to eat."

It is her effort to understand the rain forest as a whole - to make the linkages between, say, night and day or animal and plant - that has made Emmons one of the world's leading rain forest biologists.

"The next big advances we make in understanding tropical biology will be from scientists who work that way," says Ted Parker. "She always got me excited and made me ask questions. I started looking at plants to see what birds were around it ... it made me much more apt to wonder [about relationships within the rainforest]," he says. He works with Emmons on the Conservation International Rapid Assessment Program (RAP).

There are only a handful of scientists in the world qualified to do this work and with Emmons's reputation, this program buys credibility for the new form of science, says Conservation International's Brent Bailey, who hired Emmons for the RAP team's four South American expeditions. "If Louise says something is there, no one will dispute it," he says.

Broad-ranging career

Scientists traditionally have spent careers focusing on one species in one place, but Emmons has ranged broadly for 20 years on three continents studying numerous mammal species.

And while most scientists are tied to the lifeline - and time restraints - of university salaries, Emmons has traded financial security for the freedom to be constantly in the field, like the naturalists of a century ago. Her numerous scientific affiliations - the Smithsonian, the American Museum of Natural History, the Duke University Primate Center - are prestigious, but unsalaried.

Living a minimalist existence - in humid, buggy camps in Southeast Asia, Africa, and South America or back in Washington on her spare houseboat blocks from her office at the Smithsonian Institution - Emmons emulates her father's stamina and focus, observes her mother, Evelyn Hawkes. Mrs. Hawkes is a colorful translator of her daughter's dry personality. Arthur Brewster Emmons III, Louise's father, was a United States diplomat. He was also a pioneer of first-ascent mountain climbs around the world - neve r slowed by the loss to frostbite of all but four inches of each foot.

The extent of Louise Emmons's knowledge, say colleagues, is best seen in her recently published "Neotropical Rainforest Mammals: A Field Guide" (University of Chicago Press). It is the first of its kind for South America, and Emmons is perhaps the only person who has seen most of the hundreds of animals listed.

The book is laced throughout with details only Emmons could provide, says Dr. Wright. No one could describe the richness of something like the color of a jaguar's eyeshine - bright greenish yellow - if they hadn't seen it in the beam of his or her own headlamp, as Emmons has.

The book is important, say scientists, because specialized biologists often do not know many of the animals they encounter in the jungle.

And this is why, after years of living from one small grant to another, Emmons now finds herself in great demand. The worldwide environmental movement needs experts whose broad knowledge may help save the rain forest.

Rain forest degradation made a vivid impression on Emmons early in the 1970s, during her doctoral thesis work in Gabon, in West Central Africa. The plentiful game and fish she saw in her area of study were "eaten up" within years, and villagers "had to go 50 kilometers [31 miles] by boat to find a place where there was still game."

Rain forest destruction, she explains, is not just the chopping of trees. Plants dependent on seed dispersal by certain animals can disappear if the chain of interaction is interrupted - such as over-hunting of animals caused by population growth.

Finding how system works

"My research has not been specifically oriented to conservation, but finding out how the system works, the ecology of the tropical forest," says Emmons, whose sentences often begin boldly only to trail off as if a new thought has crowded in on her.

She best described her vision of the rain forest in Orion Nature Quarterly: "You can stand anywhere and be surrounded by hundreds of organisms that are all 'doing something,' going about their living in countless interactions - ants carrying leaves, birds dancing, bats singing, giant blue wasps wrestling with giant tarantulas, caterpillars pretending they are bird droppings, and so on."

It is this kind of imagery that hints at the spirit the layman might miss behind her no-nonsense persona and scientific papers about such things as ocelot behavior in moonlight, or fruit seed dispersal by vertebrates.

Stalked by a jaguar

But it is difficult to pry this out of Emmons herself.

Tales abound among family and friends of the tango-like tracking she did of a jaguar which turned and stalked her back to camp; of the way she casually handled the bubonic plague caught when handling rats; of grueling 24-hour treks through rugged terrain following wildcats.

"She doesn't think of a jaguar any differently than a mouse possum," observes Mr. Parker. "And she's totally puzzled by your incredulity" over her exotic experiences.

"It makes all the guys around feel insecure," he says, adding that rain forest natives initially will dismiss her because she is a woman and then, seeing her work, grow afraid of her, suspecting that she possesses supernatural powers.

This sort of initial underestimation seems to be a theme in her life. Just as she slips quietly - absorbed and unnoticed - through the jungle, she moves inconspicuously through life.

Except for muscled forearms and a sun-burnished nose that stands out against a pointilist dappling of pale skin and straw-colored hair, there's little physically to hint at the adventurous life she leads.

Her mother, Mrs. Hawkes, laughs hugely when she recalls urging private school officials not to pressure Louise onto a college track because "you may not find she is college caliber."